Natural occurring clays vary considerably in their color properties, even when produced from mines in the same locality or even from different sites in the same mine. Natural occurring kaolin clay deposits contain discoloring contaminants, for example, iron and titanium minerals. Titanium minerals in kaolin usually occur as discolored particles and these, coupled with iron oxides and other ferriferous material, are largely responsible for the yellow-brown shade of many kaolins. Often a clay is rejected as being unsuitable for commercial use solely on the basis of brightness even though its other physical properties, such as the viscosity of clay-water slurries and particle size, are within desired limits.
Much attention and research in the clay industry has focused on developing processes to increase the brightness of clays. In the water wash process, crude clay is slurried, degritted, fractionated to the desired particle size and the resulting fractions leached with zinc or sodium dithionite at a pH of 3.5-4.0 to improve both brightness and shade. Brightness improvements due to dithionite leaching can be quite substantial; however, the increase in brightness is generally insufficient to make high brightness products in the range of 90 TAPPI brightness, or to make lower brightness products when highly discolored crude clays are employed. Therefore, other processing methods, such as selective sedimentation, magnetic separation, froth flotation, and selective flocculation, have been developed for use in conjunction with dithionite to improve the brightness of the leached products. These methods can be employed to produce both standard and high brightness products from highly discolored starting materials by removing much of the iron stained titanium and ferriferrous material prior to the leaching step. Selective sedimentation, magnetic separation, froth flotation and selective flocculation are processes designed to remove the highly discolored titanium impurities and iron oxides. The dithionite leach step solubilizes and removes a portion of the amorphous ferriferrous impurities on the clay surface.
The most successful processes developed to date to improve the brightness of clays are those in which impurities, such as titanium and iron compounds, are removed by selective sedimentation, selective flocculation, more commonly referred to as Differential Flocculation of Anatase (DFA), and magnetic separation.
The selective sedimentation process is exemplified by U.S. Pat. No. 3,371,988 to Maynard et al. The process of this patent has been found to be disadvantageous in that relatively long sedimentation periods are required.
U.S. Pat. No. 3,857,781 to Maynard exemplifies the DFA process which is a substantial improvement over U.S. Pat. No. 3,371,988. This process involves the use of an anionic, high molecular weight polymer in the presence of specific inorganic salts which flocculates and settles out much of the iron-stained titanium dioxide contaminates at extremely rapid sedimentation rates.
A third process exemplified by U.S. Pat. No. 4,424,124 to Iannicelli involves the removal of iron oxides and iron stained TiO.sub.2 impurities by magnetic separation. All three processes, selective sedimentation, DFA, and magnetic separation produce a substantial amount of reject material which have heretofore been discarded.
A fourth process, not yet widely used, is called froth flotation which utilizes frothing agents to remove TiO.sub.2 impurities. These froths rise to the surface and are removed to leave an improved brightness product.
The present invention is directed to a process for producing a usable pigment from the reject material from the above processes or a similar process which produces a low brightness, titanium dioxide-rich reject.